1. Field of the Invention
The present invention relates to a method for forming solid shaped bodies from different inorganic powder materials such as metal powder, ceramic powder, etc. More particularly the invention relates to such a method wherein either homogenous, dense, solid bodies or porous, solid bodies are formed from an aqueous slurry containing the powder and a protein substance which is capable of forming a gel on heating.
2. Description of the Related Art
Several methods are known for forming shaped bodies, both for forming homogenous, dense bodies and porous bodies, from different inorganic powder materials. The simplest wet-forming method for dense bodies is the slipcasting method. As a rule, no binding substance is required for slipcasting and the method is used for fairly simple designs without too great demands on precision, such as sanitary products.
Other forming methods used, for example for the production of technical products and constructional products such as cutting tools, wear parts, threadguides, piezo activators, etc., may require binders to make the green bodies hold together sufficiently to be removed from molds and/or transferred to the next step, which for products of powders of the type considered usually is a sintering at temperatures of from 800.degree. C. to 2200.degree. C., depending inter alia on the melting point of the powder. In addition to a binding agent, the slurry from which the green bodies are formed usually also contains a dispersing agent for the powder. Shaped, porous and foamed, solid bodies have traditionally been prepared by use of an organic preform with foam structure which has been dipped into a powder slurry. The obtained structure has then been dried, the preform burnt off and the product finally sintered. According to more recent technique, use is made of a polymer which reacts with the water in the slurry forming carbon dioxide which leads to foam formation and possible crosslinking. The porous bodies are particularly useful in applications such as heat insulating materials, filters and catalyst supports.
Several different types of binders, such as waxes, synthetic and natural polymers, for example polyvinyl alcohol, ethylene-acrylic acid copolymers and cellulose derivatives such as methyl cellulose and hydroxyethyl cellulose, have been suggested and used to strengthen green bodies. It is also known to form green bodies using gel forming substances, for example agarose, which forms a gel on cooling, or a synthetic monomer or monomer mixture, e.g. of mono- and difunctional acrylamides and of di- and multifunctional acrylates, which polymerizes and crosslinks on heating, to form a polymer gel matrix in which the powder is distributed. Further, the use of some proteinaceous substances for the formation of green bodies is known. Some protein substances, such as gelatin and casein, have been used or suggested for use as a conventional binder material in aqueous solution.
In the European patent application 0 087 160 the use of a "gelling agent" and of a "stabilizing agent" in the preparation of porous, metallic bodies is disclosed. According to this application, a "gelling agent" which is carboxymethyl cellulose, polyvinyl pyrrolidone, polyvinyl alcohol, albumin or agar is used, and its presence is said to be necessary so that the foam structure is not destroyed when the metal powder is introduced. However, in order to solidify the foam structure a "stabilizing agent", preferably a polyurethane, formed from monomers incorporated in the foam is necessary. In this process the so called gelling agents are consequently only used for foam formation while the actual formation of a rigid gel is obtained by the polyurethane.
Gelatin has also been suggested for use as a gel former. Thus, U.S. Pat. No. 5,279,994 discloses a process for tapecasting using gelatin for gel formation. The use of gelatin for this purpose necessitates heating the entire slurry to dissolve the gelatin and cooling for setting it into a gel.
The Japanese patent application 94-226718 likewise discloses use of gelatin for its gel forming properties. According to this document a body is formed by casting a powder slurry containing the gelatin, dissolved by heating, into a mold and cooling for gelation. It is mentioned in the document that other proteins could be used, e.g. albumin, but the disclosed process in itself is strictly one using heating for dissolution of the protein and cooling for the gelation, i.e., conditions which are required for gelating but would not necessarily be gel-forming conditions for other protein materials.
Gel formation using synthetic polymers is often disadvantageous in that fairly high temperatures are required for formation of a polymer matrix and also in that fairly high temperatures are required for removal of the organic substance. Use of gelatin as a gel former has drawbacks in that the gel formation is reversible and in that heating of the entire slurry is required which involves a high energy consumption. Further, gelatin leads to a thickening of the slurry which causes difficulties in handling.